This invention pertains generally to optical apparatus and particularly to apparatus of such type having a relatively narrow field of view.
It has been known in the optical art for many years that the size of the field of view, meaning the magnitude of the solid angle visible through an optical instrument, may be controlled by use of an appropriate light baffle. For example, a relatively narrow field of view is provided in some signaling devices by installing a light at the base of a hollow tube. The field of view of such a device then is defined (in radians) by the ratio of the diameter of the hollow tube to its length.
When the field of view of an optical instrument designed to receive light (meaning any electromagnetic energy having a wavelength between that of radio frequency energy and X-rays) is to be limited, alternative approaches other than using light baffles may be used. That is, advantage may be taken of the characteristics of lenses and mirrors which are usually elements in optical instruments to restrict the field of view. For example, in an astronomical telescope wherein the focal length of an objective lens, or mirror, must be relatively long to minimize aberration, a relatively narrow field of view is a necessary condition for operation.
In X-ray technology it is well known that so-called "collimators" may be used. For example, in the so-called "Anger camera" (described in U.S. Pat. No. 3,011,057) a thick block of radiation-opaque material, say lead, may be perforated to form a number of parallel elongated collimation channels which allow only radiation parallel to such channels to pass. Because the channels are, however, formed by machining, the diameter of each is far, far greater than the wavelength of the radiation to be passed. It follows, therefore, that no practical effects of diffraction are experienced. On the contrary, however, when optical (including infrared) wavelengths are to be passed the diameters of the channels used for X-rays are such that appreciable diffraction may be experienced. For this reason, no collimators of the type used for X-rays have ever been used for optical wavelengths. Another point should be made about collimators used for X-rays: That is, there has been no recognition in the art that the interior surfaces of the channels have any significant effect on the efficacy of such channels to collimate X-rays. This failure probably arises from the fact that the materials from which X-ray collimators are made reflect only those X-rays which almost graze the interior surfaces of the channels. It follows, then, that reflections from interior surfaces are of no moment. On the other hand, at optical frequencies, reflection of rays at almost any angle to the longitudinal axis of a channel may allow light energy to pass through to the great detriment to the field defining qualities of a channel.
To reduce the effects of reflection from channel walls, the inventor here has previously taught that channels may be made by using clad fibers wherein the index of refraction of the cladding material is lower than the index of refraction of the core material. The field of view of each such channel, then, is defined by the ratio of the diameter of the core to the length of the core. Light falling on the interface between the core and the cladding material at an angle greater than the critical angle is refracted so as to be trapped within the cladding material. By choosing materials having indices of refraction which result in a very small critical angle, the effects of reflection may be reduced drastically even though the surfaces involved ordinarily are specular reflectors. While a large number of channels made in the manner just described may be joined together to form a "channel plate" which has application in an electrooptical device such as an image intensifier, the resulting assembly is expensive to build. Further, losses are experienced by reason of the basic fact that the core material is glass.
Another problem in the art, especially in the art of optical seekers for guided missiles, is that scattering due to diffuse reflection in the atmosphere limits the maximum contrast between a target in a field of view and the background within such field. The diffuse reflections from the background constitute noise with which the energy from the target must compete. There is no satisfactory remedy known for this effect.